The optical card, which records information according to the thermal input due to incident light energy, permits a considerably higher recording density than the magnetic card and, as such, is regarded as one of the important information recording media for the future.
The optical card is typically a card measuring about 85.5 mm by 54.0 mm, with a thickness of about 0.8 mm, and is available in several types, such as read-only type and read-and-write type.
Such optical card comprises a protective layer having a thickness of about 0.4 mm and disposed on the reading/writing side thereof, an information recording layer having a thickness of several .mu.m and disposed behind said protective layer, and a substrate or base member disposed behind said information recording layer, all laminated into an integral unit. Since the base member is a mere supporting means, it need not have special optical properties but the protective layer must be transparent and optically isotropic. The material heretofore used for this protective layer is a sheet of polyvinyl chloride, polymethyl methacrylate or polycarbonate. For protection against damage, a hardened surface layer is sometimes disposed on top of the protective layer.
An exemplary laminar construction for the read-only optical card is as follows: from readout side, hardened injury-proof surface layer/protective layer/information recording layer/adhesive layer/base member. The information recording layer may, for example, be a two-layer structure consisting of a non-silver salt pattern layer and an aluminum reflective layer.
An exemplary laminar construction of the read-and-write optical card is as follows: from readout side, hardened injury-proof surface layer/protective layer/information recording layer/base member/protective layer. The information recording layer may, for example, be a two-layer structure consisting of a crust layer (silver salt pattern layer) and a sub-layer.
The protective layer to be used for protecting the recording layer of an optical card must satisfy the following requirements: it should be transparent and optically isotropic (with a small birefringence), of high mechanical strength, hard and non-yielding, low in thermal conductivity, heat-resistant, resistant to the solvent used in the application of a recording medium, and least hygroscopic. Furthermore, it must not be too costly to frustrate the commercial implementation of the product.
However, the optical card comprising a polyvinyl chloride layer as the protective layer and an information recording layer disposed thereon has the disadvantage that even if the retardation value of the polyvinyl chloride layer is set to a predetermined value, this value is frequently altered by exposure to the solvent used in the formation of the information recording layer as well as by thermal history, thus failing to provide an optical card with stable performance.
The optical card carrying an information recording layer on a polymethyl methacrylate sheet has the disadvantage that no means is available for preventing absorption of moisture by the polymethyl methacrylate base and that this material is inadequate in solvent resistance.
The optical card carrying an information recording layer on a polycarbonate sheet also has the disadvantage that this material is liable to be dissolved by the solvent used in the formation of the recording layer.
The object of this invention is to provide a laminate which is suitable for use as a protective layer for the information recording layer.